1. Field of the Invention
The present invention relates to a rectifier arrangement of an alternator to be mounted in a passenger car, a truck or the like.
2. Description of the Related Art
In order to reduce vehicle running resistance and to increase space provided in the passenger compartment, engine compartments have been reduced in size. Accordingly, temperature in the engine compartment and of an electric device such as an alternator has become higher. Moreover, as a vehicle has been equipped with various safety devices, the output power of the alternator has increased. This has narrowed the space of the engine compartment. Thus, it is necessary to provide an alternator that generates higher power without increasing its size.
On the other hand, as the engine has become more powerful and compact, vibration of the engine has increased. Thus, such a vehicle requires a compact, powerful, temperature resistant, vibration resistant and inexpensive alternator.
FIG. 12 illustrates a portion of a conventional alternator having an ordinary rectifier disposed in a space S between frame 100 and metal cover 101. The rectifier is composed of terminal plate 104, plus potential cooling fin 105 (hereinafter referred to as plus fin), bushing 106 and minus potential cooling fin 107 (hereinafter referred to as minus fin). Pipe rivet 103 fastens terminal plate 104, plus fin 105, bushing 106 and minus fin 107 in this order. Plus fin 105 has plus rectifier elements (not shown), and minus fin 107 has minus rectifier elements. The rectifier elements are fixed to fins 105, 107 respectively by soldering or press-fitting. Metal terminal 110 is molded integrally with terminal plate 104 to form a rectifier circuit and is connected to lead 109 which extends from a stator winding at one end and to one of the rectifier elements at the other end. Terminal plate 104 is molded with a thermoplastic resin such as polyphenylene sulfide. Bushing 106 is disposed between two fins 105, 107. Bushing 106 is made of an inexpensive thermosetting resin such as a phenolic resin because bushing 106 is not required to be molded with a metal member and is simple in shape. The rectifier is fixed to frame 100 together with cover 101 by a bolt 102 that is press-fitted to frame 100 and nut 108. Bolt 102 is inserted into pipe rivet 103 and a mounting hole of cover 101. Thus, plus fin 105 is spaced apart from minus fin 107, frame 100, bolt 102 and cover 101, which are fixed together.
When ambient temperature and temperature of rectifier in high power operation are high, creep--a gradual increase in strain due to viscoelasticity of the resin--may take place on thermoplastic terminal plate 104, resulting in that the resinous member shrinks in the direction to loosen nut 108. This reduces the fastening force of the rectifier, thereby causing breakdown of the rectifier elements and leads 109 of the stator winding.
JP-A-6-133509 proposes an improved structure, in which minus rectifier element is press-fitted to the frame, thereby omitting a minus fin. A bushing is made of thermoplastic resin such as polyphenylene sulfide to have a rectifier-forming metal terminal molded together.
Because the axial length of the bushing around the nut is comparatively thick, the strain increases due to the creep to loosen the nut. Because the minus rectifier element is fixed to the frame, cooling air windows can not be formed to be wide enough to cool the plus fin. This accelerates the creep further. The bushing must be provided for each engagement hole to enclose and insulate the plus fin completely. Moreover, because of complexity in shape, tooling and manufacturing costs increase. Further, polyphenylene sulfide is generally expensive, thereby increasing material costs.